The present invention relates to ink jet printers and, more particularly, to a printing apparatus and a printing method in which drops from at least one jet drop stream carry electrical charges of either polarity and are deflected in either of two directions as they subsequently pass through a static electric deflection field.
Ink jet printers such as shown in U.S. Pat. No. 4,085,409, issued Apr. 18, 1978, to Paranjpe, are known in which the drops in one or more jet drop streams are selectively electrically charged at the time that the drops are formed. The drops are formed from a fluid filiment which emerges from a print head. The jet drop stream is directed toward a moving print receiving medium, but an electric field is provided in the path of the jet such that highly charged drops are displaced laterally and directed to strike a drop catching device. The uncharged drops and the drops carrying lesser charges are not deflected or are deflected only slightly by the field and therefore pass through the field, striking the print receiving medium. The Paranjpe patent shows a multiple jet printer in which each jet drop stream services a number of print positions on the print receiving medium due to the drops being charged to a number of different charge levels and therefore deflected in varying amounts.
The jet drop streams are typically formed in an ink jet printer by supplying ink under pressure to the fluid reservoir of a print head. The print head defines a number of orifices, communicating with the fluid reservoir, from which the fluid filiments emerge. Typically, the fluid filiments are mechanically stimulated so as to break up into the streams of drops of uniform size and spacing.
Charging of the drops in a stream is accomplished by positioning a charge electrode adjacent the point of drop formation of the stream and impressing upon the charge electrode an electrical potential which differs from the electrical potential of the fluid filiment. As a consequence, a concentration of electric charge is formed on the tip of the fluid filiment and this charge is carried away by the next formed drop as it breaks from the filiment.
It will be appreciated that accurate drop placement on the print receiving medium is dependent, in part, upon accurate charging of the drops. One source of charge level error is the drops which have been previously formed in the stream. Assuming a drop in the stream carries a charge, the charge induced on the next drop to be formed will be affected both by the voltage on the charge electrode and by the electric charge carried by the previously formed drop. It will be appreciated that the previously formed drop will tend to induce a charge of opposite polarity in the following drop. The previously formed drop will thus offset, to some extent, the charging effect of the voltage applied to the charge electrode. The resulting error in drop charge level produces a deflection of the drop which is less than or greater than anticipated and, as a consequence, a misplacement of the drop on the print receiving medium.
This phenomenon, termed drop-to-drop cross talk, has been compensated in prior art devices by inserting "guard drops" between successive print drops. The guard drops space successive print drops further apart and thereby reduce the drop-to-drop cross talk between print drops. U.S. Pat. No. 3,562,757, issued Feb. 9, 1971, to Bischoff shows a jet drop device in which guard drops are provided between the drops available for deposit on the print receiving medium, with all of the guard drops being uncharged. These uncharged drops act as a shield between successively formed charged drops such that a previously formed charged drop does not adversely affect the level of charge carried by a subsequently formed drop.
U.S. Pat. No. 3,833,910, issued Sept. 3, 1974, to Chen, discloses an ink jet printer in which guard drops are provided between successive print drops. Every alternate drop is selectively charged as necessary for use in printing, and each intervening guard drop is charged with an opposite polarity charge which is proportional to the charge on the preceding print drop. As a result, the cross talk effect from a preceding print drop on the next formed print drop is effectively canceled by the opposite polarity cross talk effect from the intermediate, charged guard drop.
It will be appreciated that a large number of other factors affect the accuracy with which drops of ink are deposited on a moving print receiving medium. Such factors, among others, include fluctuation in the masses of the drops produced in the jet drop streams; fluctuations in stream velocity due to changes in ink temperature, pressure, and viscosity; manufacturing error in the position and straightness of the orifices; inaccuracy of the timing of the charge electrode voltages; fluctuations in the velocity of the moving print receiving medium; and the fluctuations in stream velocity due to the aerodynamic effects of drops upon each other among others.
It has been recognized, however, that some of these sources of error can be minimized by reducing, to as large a degree as practical, the amount of deflection of the print drops. U.S. Pat. No. 4,060,804, issued Nov. 29, 1977, to Yamada shows an ink jet printer having two ink jet nozzles. Each nozzle prints along a separate band of the print receiving medium, with the bands abutting along a "seam". Yamada recognizes that the accuracy of drop placement is most important along the seam, since errors here in drop placement will be readily apparent to the observer as discontinuities in the printed image. In order to provide for the least amount of error, Yamada recognizes that the highest accuracy of drop placement results from deflecting the print drops by the least amount and, therefore, prints adjacent the seam with drops from each of the nozzles which are deflected by the minimum amount necessary to clear the drop catchers. It will be appreciated, however, that the Yamada disclosure relates to an ink jet printer construction which is uniquely limited to a two-nozzle design and, further, that the improved drop placement accuracy is effected only along one edge of each of the bands.
U.S. Pat. No. 3,596,275, issued July 27, 1971, to Sweet, in FIGS. 11, 12a, 12b, and 13, discloses an arrangement in which pairs of guard drops intermediate successive single print drops receive a sufficiently high charge such that they are deflected by a static electric deflection field to a catcher. The drops which are used for printing, as seen in FIG. 12a, are apparently charged in a bipolar fashion, since they are deflected toward either the positive or negative deflection electrode. The Sweet disclosure relates to a single jet printer in which the drops from the jet are required to be deflected substantially in order to strike the print positions which are widely spaced across the print receiving medium. As indicated by FIG. 11, and by the accompanying description, the device operates as an oscillograph; it receives a bipolar, fluctuating electrical charge signal and prints a curve representative of the fluctuations in this signal.
Accordingly, it is seen that there is a need for an ink jet printer in which the effects of drop-to-drop cross talk within a jet are compensated and, additionally, in which the accuracy of drop placement on the print receiving medium is improved by deflecting the drops only slightly.